Page:EB1911 - Volume 21.djvu/193

Rh PERMEAMETER, an instrument for rapidly measuring the permeability of a sample of iron or steel with sufficient accuracy for many commercial purposes. The name was first applied by S. P. Thompson to an apparatus devised by himself in 1890, which indicates the mechanical force required to detach one end of the sample, arranged as the core of a straight electromagnet, from an iron yoke of special form; when this force is known the permeability can be easily calculated. (See )

PERMIAN, in geology, the youngest and uppermost system of strata of the Palaeozoic series, situated above the Carboniferous and below the Trias. The term “Permian” (derived from the

Russian province of Perm, where the rocks are extensively developed) was introduced in 1841 by Sir R. I. Murchison. In England the series of red sandstones, conglomerates, breccias and marls which overlie the Coal Measures were at one time grouped together in one great formation as the “New Red Sandstone,” in contradistinction to the Old Red Sandstone below the Carboniferous: they were likewise known as the Poikilitic series (from Gr. , mottled) from their mottled or variegated colour. They are now divided into two systems or groups of formations; the lower portion being included in the Palaeozoic series under the name Permian, the upper portion being relegated to the Mesozoic series and termed Trias. In Germany the name Dyas was proposed by J. Marcou for the rocks of this age on account of the twofold nature of the series in Thuringia, Saxony, &c. The intimate stratigraphical relationship that exists in many quarters between the Permian rocks and the Carboniferous beds, and the practical difficulties in the way of drawing a satisfactory base-line to the system, have led to the adoption of the term Perma-carboniferous in South Africa, southern Asia, America, Australia and Russia, for strata upon this horizon: C. W. von Gumbel used “Post-carbon” in this sense. In a similar manner Perma-triassic has been employed in cases where a stratigraphical passage from rocks with Permian fossils to others bearing a Triassic fauna is apparent.

The Permian system in England consists of the following subdivisions:—

From the thicknesses here given it is evident that the Permian rocks have a very different development on the two sides of England On the east side, from the coast of Northumberland southwards to the plains of the Trent, they consist chiefly of a great central mass of limestone. But on the west side of the Pennine Chain, and extending southwards into the central counties, the calcareous zone disappears, and we have a great accumulation of red, arenaceous and gravelly rocks.

The lower subdivision attains its greatest development in the vale of the Eden, where it consists of brick-red sandstones, the Penrith sandstone series, with some beds of calcareous conglomerate or breccia, locally known as "brockram," derived from the waste of the Carboniferous Limestone. These red rocks extend across the Solway into the valleys of the Nith and Annan, in the south of Scotland, where they lie unconformable on the Lower Silurian rocks. Their breccias consist of fragments of the adjacent Silurian greywackes and shales, but near Dumfries some calcareous breccias or “brockrams” occur. These brecciated masses have evidently accumulated in small lakes or narrow fiords Much farther south, in Staffordshire, and in the districts of the Clent and Abberley Hills, the brecciated conglomerates in the Permian series attain a thickness of 400 ft. They have been shown by Sir A. C. Ramsay to consist in large measure of volcanic rocks, grits, slates and limestones, which can be identified with rocks on the borders of Wales. Some of the stones are 3 ft. in diameter and show distinct striation. The same writer pointed out that these Permian drift-beds cannot be distinguished by any essential character from modern glacial drifts; on the other hand, W. W. King and others have opposed this view. The middle subdivision is the chief repository of fossils in the Permian system. Its strata are not red, but consist of a lower zone of hard brown shale with occasional thin limestone bands (Marl Slate) and an upper thick mass of dolomite (Magnesian Limestone). The latter is the chief feature in the Permian development of the east of England. It corresponds with the Zechstein of Germany, as the Marl Slate does with the Kupfer-schiefer. It is a very variable rock in its lithological characters, being sometimes dull, earthy, fine-grained and fossiliferous, in other places quite crystalline, and composed of globular, reniform, botryoidal, or other irregular concretions of crystalline and frequently internally radiated dolomite. Though the Magnesian Limestone runs as a thick persistent zone down the east of England, it is represented on the Lancashire and Cheshire side by bright red and variegated sandstone covered by a thin group of red marls, with numerous thin courses of limestone, containing Schizodus, Bakevellia and other characteristic fossils of the Magnesian Limestone.

Concerning the rocks classed as Permian in the central counties of England there exists some doubt, for recent work tends to show that the lower parts are clearly related to the Carboniferous rocks by their fossils; while there is little evidence to warrant the exclusion of the higher beds from the Trias. Similarly in south Devon, where red sandstones and coarse breccias are well exposed, it has been found difficult to say whether the series should be regarded as Triassic or Permian, though the prevailing tendency is to retain them in the latter system.

The “Dyas” type of the system is found in enormous masses of strata flanking the Harz Mountains, and also in the Rhine provinces, Saxony, Thuringia, Bavaria and Bohemia. In general terms it may be said that in this region there is a lower sandy and conglomeratic subdivision with an upper one more calcareous; the former is known as the Rothliegende, the latter as the Zechstein group On the south side of the Harz Mountains the following subdivisions are recognized:-

The name Rothliegende or Rothtodtliegende (red-dead-layer) was given by the miners because their ores disappeared in the red rocks below the copper-bearing Kupfer-schiefer. The Kupferschiefer, although so thin, has been worked in the Mansfeld district for a long period; it contains abundant remains of fish (Palaeoniscus, Platysomus) and plants (Ullmannia). The beds of rock-salt in the German Zechstein are of the greatest importance; at Sperenberg near Berlin it has been penetrated to a depth of 4000 ft. Associated with the salt, gypsum and anhydrite are numerous